I want to use split toning to grade my image. Is it better to create good contrast in my photo before using split toning OR to decrease contrast in my photo by making it more flat prior to split toning?

Edits in Lightroom are independent of each other, meaning the final result is the same regardless of the order in which the edits are made. The choice is up to you. You can decide if split toning is easier for you to see without the added contrast or with it. My experience is that there is always a little back and forth final tweaks require before the image looks the way I want it.